In such a case, it is necessary for the voltage of the source and the voltage of the battery when discharging to be practically equal, since they feed the load alternately. This poses a problem for charging the battery and in particular when it is of a type which it is preferable to charge at a constant current rather than at a constant voltage. This is the case, for example, with sealed nickel-cadmium batteries.
The preferred way of solving the problem is to divide the battery into two parts which are charged in parallel but which are discharged in series so that the voltage supplied by the electricity supply is always higher than the voltage across the batteries during charging.
Particularly for vehicle engine-starting applications, arrangements are already known in which batteries are charged in parallel and discharged in series. This is the case, for example, of the system described in U.S. Pat. No. 2,671,978. Most of these systems operate manually under the control of a knob which is operated by the driver.
French Pat. No. 1 505 032, more particularly, describes an arrangement which operates automatically; however, it relates to an arrangement in which the terminals between which the charger is connected and those to which the load is connected are distinct. The automatic device described in British Pat. No. 1 428 661 has pairs of common terminals, but it has the disadvantage of high permanent power consumption, the charging resistors remaining permanently connected.